The field of radio communications has evolved substantially in order to facilitate various protocols and to address preferences in radio device design. A typical communication device now includes multiple radio transceivers and receivers for various different radio protocols, and users have come to expect being able to communicate over various networks and to have local and personal range connectivity. At the same time, users have preferred smaller devices as they are easier to carry. Accordingly, manufacturers have responded by integrating communication circuitry and reducing the size and weight of communication devices.
One area where meeting user expectations for functionality and form factor has been challenging is in portable two-way radio devices. These devices are used for near-instant communication (i.e. “push to talk”) and because of that capability they remain the primary choice for communications among organizations such as police, fire, rescue, and other organizations where rapid communication is desirable. Two-way communications systems have evolved over time, and are now conducted in VHF, UHF, and 800/900 MHz bands. Furthermore, there has been a desire to include certain data communication functionality in these devices as well, such as, for example, the Long Term Evolution (LTE) standard in 700 and 750 MHz bands, among others.
Integrating communications circuits for multiple bands has been accomplished by use of multiple front ends, with one front end for each band, and where each front end is entirely operated in a voltage mode. Separate front ends are used, in part, due to noise considerations while operating in the voltage mode, as well as inherent bandwidth limitations. The desire to reduce the size of portable device has been somewhat enabled by the use of lithium-based battery cells. Rather than using multiple cells connected in series, a single lithium ion battery cell, for example, can provide 3-4 volts, which is sufficient to operate most of the circuitry in a portable device. However, for some RF circuits operating in a voltage mode, a higher voltage is needed. Accordingly, the battery voltage can be stepped up using a switching regulator (e.g. a boost mode regulator). However, switching noise then has to be dealt with, or it can adversely affect receiver sensitivity.
Alternatively, some manufacturers have explored using a current mode front end, where the received signal is converted to a corresponding current signal. This approach has the benefit of being operable at low voltage levels, obviating the need for a voltage boosted in systems powered by a signal battery cell. Furthermore, a current mode front end can be designed to have a low noise figure and is operable over a broader frequency range. However, gain control is typically performed in stages after the current mode stage, in a voltage control mode. It is possible to use elements of the transconductance amplifier for feedback but this has shown to cause DC jumps in the signal level. Furthermore, the die area required to implement even an 8 bit control would be substantial.
Accordingly, there is a need for a method and apparatus for automatic gain control in current mode for a current mode front end of a radio receiver.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.